Theoretical study of carbon-halogen bond dissociation enthalpies of substituted benzyl halides. How important are polar effects?

It has been suggested (Clark, K. B.; Wayner, D. D. M. J. Am. Chem. Sec. 199 1., 113, 9363-9365) that C-Br bond dissociation enthalpies (BDEs) in 4-YC6H 4CH2-Br decrease as Y becomes more electron withdrawing because of increasi ng destabilization of the Cdelta+-Brdelta- dipole and, furthermore, that th e direction and magnitude of the effect of Y on 4-YC(6)H(4)Z-X BDEs could b e correlated with the sign and magnitude, respectively, of the electronegat ivity difference between Z and X. The results of density functional theory (DFT) calculations using a locally dense basis set approach with the B3LYP functional and 6-311+G(2d,2p) as the primary basis set on 4-YC6H4CH2-X, for X = H, Br, Cl, and F with Y = NH2, HO, CH3O, CH3, H, CF3, CN, NO2 and BH2, show that the effects of Y on CH2-X BDEs are small (less than or equal to 2.0 kcal/mol) for all our classes of compounds and are roughly equal for ea ch Y for the three halides. Furthermore, almost all Y's reduce CH2-X BDEs r elative to Y = H. Clark and Wayner's intriguing hypothesis that the magnitu de of the effects of Y on 4-YC(6)H(4)Z-X BDEs depends on the magnitude of t he differences in the electronegativities of Z and X should be discarded.